The lick starts on the root note, A. It continues with a pair of hammer-ons from the minor 3rd. The second hammer-on finishes on the major 3rd. This kind of minor/major sound gives a very characteristic blues sound.

Finally, the root A note is played again. Let this note ring on and give it some vibrato effect. The root A notes at the beginning and the end of the lick work with the major 3rd to establish the key.